There are many applications where it is necessary to provide a switch for radio frequency signals. One such application is in a radio transceiver where it is desired to selectively switch an antenna to either the transmitter or receiver thereof.
Several techniques for providing an antenna switching function are known. One such system employs a pair of magnetically operated reed switches, one of which is normally closed for connecting the receiver to the antenna and another which is normally open for connecting the antenna to the transmitter. The reed switches are operated by means of a permanent magnet or electromagnet. This arrangement suffers from the disadvantage of utilizing large and bulky, high cost, fragile components corresponding in a loss of reliability.
Solid state switches provide an attractive alternative to the foregoing electromechanical arrangement. Two solid state alternatives are known. However, one suffers from limited power handling capability, while the other suffers from severe bandwidth limitations.
FIG. 1 illustrates a common PIN diode switching network. In the transmit mode the transmitter voltage V.sub.TX is coupled to the supply voltage while the receive voltage V.sub.RX is coupled to ground. The transmit voltage V.sub.TX forward biases diode CR.sub.TX and, thereby, enables the transmitter to antenna path T.sub.X -A.
This configuration is very broadband since the forward path T.sub.X -A includes only a forward biased PIN diode CR.sub.TX, while the disabled path A-R.sub.X is isolation is provided by a reverse biased diode CR.sub.RX.
However, note that the disabled path diode CR.sub.RX can only be reverse biased by the maximum available supply voltage minus one diode drop. The major drawback is that whenever the peak RF swings at the output node N exceed the available supply voltage, the RF signal will cause the disabled diode CR.sub.RX to turn on allowing power into the path intended to be disabled, thereby eliminating the transmitter to receiver isolation T.sub.X -R.sub.X. In land-mobile communications, the maximum available supply voltage is 12 volts or less, while transmit power can approach 150 watts and transmit voltage swings can exceed 100 volts (&gt;&gt;12 volts). Clearly, the limited power handling capability of this configuration makes it unacceptable for land-mobile applications.
An alternative configuration is illustrated in FIG. 2. In the transmit mode, both PIN diodes CR.sub.TX and CR.sub.RX are forward biased by the supply voltage V.sub.TX to a conducting state. A quarter wavelength transmission line is employed to reflect the effective short CR.sub.RX as an open circuit at the output node N. However, the major limitation of this configuration is that it contains a resonant element (i.e., the quarterwave line) that severely limits the bandwidth of the network. That is, at twice the resonant frequency, the quarter wave line would reflect a short at the output node N rather than an open circuit.
The instant invention solves the above-mentioned problems of having a broadband switching network which is able to have an adequate power handling capability by developing the required reverse bias voltage from the large RF voltage signal swings to keep the receive signal path switched off.
This invention represents a significant advance over the prior art an this technical field by providing a broadband solid state antenna switch having a high power handling capability.